1. Field of the Invention
The present invention relates to a fuel injection valve for a fuel injection equipment of an internal combustion engine and, more particularly, to structure of a fuel injection valve capable of suppressing bounce at the time of opening and closing a needle (valve element).
2. Description of the Related Art
To extend the range of combustion control in an internal combustion engine, improvement in flow controllability of a fuel injection valve has been conventionally required.
As a method of extending the range of small flow rates of the controllable quantity of flow of fuel, bounce suppression of a needle (valve element) is known.
Due to the bounce of the needle, since the needle is moved in valve closing direction at the time of vale opening and is moved in valve opening direction at the time of valve closing, a problem exists, for example, in that the minimum driving time period of the needle is increased at the time of valve opening, and the minimum flow rate is increased at the time of valve closing. The adjustment of flow rates of fuel can be made depending on length of pulse signals (valve opening time period signals) to be transmitted to a fuel injection valve.
However, in a period of the unstable lift of the needle (i.e., the travel in valve opening direction of a needle) owing to the bounce of the needle at the time of valve opening, it is necessary to transmit valve opening signals at all times thus to regulate the minimum flow rate.
Further, when the bounce of the needle occurs at the time of valve closing, although it is a short time period, the valve is open again, after it has been closed, to inject excess fuel, resulting in increase of the minimum flow rate of fuel to be injected.
Furthermore, owing to the bounce of the needle, fuel is injected (secondary injection) from a gap between the needle and a valve seat.
Since this secondary injection is an uncontrollable injection, a problem exists in that abnormal combustion occurs in a cylinder of an engine, and that deposits are adhered to a fuel injection valve.
Additionally, those deposits are made by products of carbon particles and tar being produced by the combustion of fuel, and the carbon particles being adhered to the valve with tar. When deposits are adhered to an injection port of fuel, there arises a further problem such as a smaller cross-section of injection port resulting in a smaller flow rate of fuel.
Since the secondary injection is a powerless injection, fuel is likely to adhere to the injection port and to bring a factor of increase of production of the deposits.
Thus, in the field of fuel injection valves for injecting fuel to an internal combustion engine, several attempts of suppressing bounce that occurs at the time of valve opening/closing of a needle (valve element) have been heretofore proposed.
For example, the published Japanese translation of a PCT application No. 528672/2002 proposes a method, in which a buffer spring (specifically, Belleville spring annularly surrounding a needle) is inserted between an armature (moving iron core) and a stopper to cause the buffer spring to absorb the bouncing force that takes place due to inertial force of the needle at the time of valve closing of the needle, thereby suppressing the bounce of the needle.
However, the method of bounce suppression of a needle that is proposed in the published Japanese translation of a PCT application No. 528672/2002 has several problems as follows.
Reliability assurance of the sliding portion between the needle and the armature is required.
A buffer spring and members for fixing the buffer spring are added, so that the number of parts is increased, as well as the structure comes to be complicated.
Since suppression effects of bounce depend on the weight of the armature and the buffer spring at the time of valve closing, and on the weight of the needle and the buffer spring at the time of valve opening, in the case where the weight of the armature and the weight of the needle are different, the setting range of bounce suppression effects is restricted.
Since an inertial force of the needle is absorbed by the needle and the buffer spring, the needle is lifted not less than a travel amount having been set, resulting in worse controllability of flow rate.
Further, the Japanese Patent No. 3723800 (Patent Document 2) proposes another method, in which there are provided a passage that communicates with an air gap formed between an armature and a core (fixed iron core) , and a volume chamber that communicates with the air gap via this passage; and the mentioned passage and volume chamber are set to be of such configuration dimension as to phase-invert pressure waves that occur in the air gap to return it to the air gap, and to make the pressure in this air gap larger, thereby suppressing the bounce of a needle.
In addition, the Helmholtz resonator is formed with the mentioned passage and pressure chamber.
However, the mentioned bounce suppression method of the needle that is proposed in the Japanese Patent No. 3723800 has problems as follows.
To have the configuration of a fuel injection valve giving most bounce suppression effects, it is necessary that a pressure pulsation period of the air gap between the armature and the core is coincident with a resonance frequency of the resonator.
Therefore, by measuring pressure pulsation of the air gap between the armature and the core, the configuration of the resonator can be set in accordance with the pulsation period thereof.
Actually, between the air gap between the armature and core, and the resonator, there is a further air gap (side gap) between the armature and a holder.
Accordingly, the period of pressure pulsation having occurred in the air gap between the armature and the core is changed at the time point of reaching the resonator, thus making it hard to be the optimum design.
Since an oil path (fuel passage) facing a resonator is formed of two air gaps of one air gap between the core and armature and another air gap (side gap) between the armature and holder, a distance through which pressure is transmitted comes to be longer. Thus, the sufficient bounce suppression effects cannot be achieved.
Moreover, as a fuel pressure in use becomes higher, the mentioned disadvantage takes place more considerably.
The resonator is disposed in the holder, so that the mechanical strength (pressure resistant properties) of the holder comes to be lower.
Consequently, it is necessary to prevent the worse pressure resistance, resulting in a larger diameter of holder, upsizing and increase in cost of the fuel injection valve.
As described above, several attempts for bounce suppression have been heretofore proposed. However, it is a recent trend that the load a fuel pressure imposed on a needle is increased due to, e.g., higher pressure of the fuel in use, wherebya problem exists in that the sufficient effect cannot be obtained with measures having been proposed.
Furthermore, it is necessary to insert a buffer spring (Belleville spring) between the armature and stopper, or to provide a pressure chamber at a place that communicates with the air gap between the core and armature, and the air gap between the armature and holder to form a resonator. Thus, a further problem exists in that the number of parts is increased, and that the structure is complicated.